CD79A and selective IgA deficiency disease: In summary, the diagnostic algorithm for celiac disease begins with the determination of tTG-IgA and total IgA in patients on a normal gluten diet, followed by duodenal biopsy in adults (or biopsy-free diagnosis in children with tTG-IgA >10× URL and positive EMA), with IgG-based tests used in IgA deficiency and HLA-DQ2/DQ8 testing reserved for patients on a gluten-free diet.